Solid-state imaging readers have been used in supermarkets, warehouse clubs, department stores, and other kinds of retailers to electro-optically read one-dimensional bar code symbols, particularly of the Universal Product Code (UPC) type, on products to be purchased, each symbol having a row of bars and spaces spaced apart along one direction, and also for processing two-dimensional symbols, such as Code 49, on such products, as well as other items. The structure of Code 49, which introduced the concept of vertically stacking a plurality of rows of bar and space patterns in a single symbol, is described in U.S. Pat. No. 4,794,239. Another two-dimensional code structure for increasing the amount of data that can be represented or stored on a given amount of surface area is known as PDF417 and is described in U.S. Pat. No. 5,304,786.
A typical imaging reader has a one- or two-dimensional array of cells or photosensors, which correspond to image elements or pixels in a field of view of the array, and is similar to that used in a digital camera. The array may be a one- or two-dimensional charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device, together with associated circuits for producing electronic signals corresponding to a one- or two-dimensional array of pixel information over the field of view. A microprocessor is used to analyze and decode the captured image of the symbol. The array may be used for capturing a monochrome image of a symbol as, for example, disclosed in U.S. Pat. No. 5,703,349. The array may have multiple buried channels for capturing a full color image of a target as, for example, disclosed in U.S. Pat. No. 4,613,895. It is common to provide a two-dimensional CCD with a 640×480 resolution commonly found in VGA monitors, although other resolution sizes are possible.
Yet, the use of imaging readers, especially hand-held movable readers, for reading symbols has proven to be challenging. An operator cannot see exactly whether the symbol is within the field of view of the array during reading. The symbol must lay preferably entirely within the field of view to be successfully decoded and read. It is not uncommon for the operator to repeatedly move the portable reader in multiple directions and repeatedly aim the portable reader at a single symbol several times before an indicator advises the operator that the symbol has been successfully read, thereby slowing down transaction processing and reducing productivity.
This blind aiming at the symbol is easier if the symbol is relatively small or is far away from the reader, because then the chances that the symbol will lay within the field of view are greater. However, in most cases, this blind aiming at the symbol is difficult to overcome, especially when the position and orientation of the symbol are variable. The symbol may be oriented in a “picket fence” orientation, in which the elongated parallel bars of the one-dimensional UPC symbol are vertical, or in a “ladder” orientation, in which the symbol bars are horizontal, or at any orientation angle in between.
To help overcome this blind aiming problem, an aiming light arrangement is typically mounted in the reader, for projecting a visible aiming light pattern to visually target the symbol and, thus, advise the operator which way the reader is to be moved in order to position the aiming light pattern on the symbol, typically at the center thereof, prior to reading. As advantageous as such aiming light arrangements are, they have proven to be less than satisfactory in certain situations. For example, in a brightly lit environment illuminated by indoor lighting of high intensity, or by outdoor lighting such as sunlight, it is often difficult for the operator to see the aiming pattern, because the aiming pattern is “drowned” out by the bright ambient light.
To improve the visibility of the aiming pattern, lasers are sometimes used as the sources for the aiming lights. A laser beam of high output power is desirable for an increased ambient light immunity and greater aiming pattern visibility. However, for safety reasons, there are regulations concerning the maximum level of laser beam output power that can be emitted from an electro-optical reader. For example, a Class II laser is limited to a maximum laser output power of 1 milliwatt if there is no laser shut-off feature. If there is a laser shut-off feature, then a higher power in the emitted laser beam is permitted. However, it is not always possible to increase the laser beam output power enough to have an aiming light pattern that is easily visibly discernible by the operator, especially in a brightly lit environment, due to laser safety restrictions.